Such a roll rotates integrally with an axle that is joined to the roll by means of a key.
Said rolls are illustrated in FIGS. 1 to 3. A key way 2 of the roll 1 extends from one side to the other side thereof.
FIGS. 2 and 3 illustrate the roll 1 assembled with a driving axle 3 under the rolling process. A key 5 is inserted into a cavity defined by the key way 2 of the roll 1 and a key way 4 of the driving axle 3. Thus, the roll 1 rotates integrally with the driving axle 3 to roll a work piece 7.
Reference numerals 6 and 8 denote internal stresses around the key way 2 of the roll 1. As shown in FIG. 2, when the driving axle 3 rotates, the key 5 pushes a front side of the key way 2 to rotate the roll 1 together with the axle 3. Therefore, a compressive stress 6 acts around the front side of the key way 2. As the key 5 approaches the work piece 7, the deformation resistance of the work piece 7 works in a direction of obstructing the movement (i.e., the rotation of the roll) of the key 5, which results in increasing the compressive stress 6 around the front side of the key way 2.
Further, as shown in FIG. 3, when the key way 2 moves away from the work piece 7 by further rotating the roll 1, a tensile stress 8 acts around a rear side of the key way 2, while the compressive stress 6 acts around the front side of the key way 2. Specifically, since a frictional force between the roll 1 and the work piece 7 acts in a direction of obstructing the movement of the key 5, the tensile stress 8 strongly acts around the rear side of the key way 2. Metals with high tensile strength (e.g., steel) are conventionally used for the rolls.
The outer surface of the roll, which contacts the work piece, is subject to compressive and thermal stresses that repeatedly act thereon. When the surface of the roll is worn out due to such stresses or torn off by fatigue cracks, the quality of the rolled surface significantly deteriorates and the rolling process must be stopped for maintenance or repair. To prevent such problems, it is desirable that the roll has high wear/thermal/fatigue resistances.
A cemented tungsten carbide (CTC), including tungsten carbide (WC), has good wear resistance and high temperature mechanical properties. Thus, when a roll made of a tungsten carbide is used, the surface quality of rolled products is improved and the rolling speed is increased, compared to a metallic roll made of carbon steel, etc. Carbide rolls may be used for hot rolling to produce iron rods or bars. Although the carbide rolls have a very strong compressive resistance, they are weak against tensile stresses. Therefore, when a carbide roll and an axle, to which a key structure such as the one shown in FIGS. 1-3 is applied, are used for the rolling process, the carbide roll may be easily broken down by high tensile stresses generated around the key way.
Thus, a carbide roll 11 without any key way is used for the rolling process, as shown in FIG. 4. The carbide roll 11 is axially pressed on both sides with bolts (FIG. 5) or hydraulic devices to transmit a driving force of the driving axle to the roll 11 through the frictional force between the roll 11 and the driving axle. In the case of the carbide roll 11, the driving force is transmitted only through the frictional force between the roll 11 and the driving axle. As such, when a high driving force is applied to the driving axle, slip may occur between the roll 11 and the driving axle, which causes a failure in transmitting the driving force. Therefore, the carbide roll 11 is rarely used in case the driving torque is over 1000 kgf·m.
Japanese Laid-Open Patent Publication No. (Sho) 59-21415 discloses a ceramic roll 21 having key ways on a side face (shown in FIG. 6). The ceramic roll 21 has linear key ways (22) along the diameter on the side face. A metallic driving axle 23 has linear protrusions 24 coupled to the key ways 22. Since the driving force between the ceramic roll 21 and the driving axle 23 is transmitted via the wide contact surface of the key ways 22 and the protrusions 24, an impact force which acts when the roll starts to move is decreased. However, since the key way of the roll 21 extends over the side face along the diameter and ending into a semi-circular concave portion at the outer peripheral surface of the roll 21, the portion of the outer peripheral surface cannot be used for rolling. This increases the size of the roll 21 and manufacturing costs. Furthermore, due to manufacturing tolerances, it may be the case that only a specific pair of the joined key way and the key (among the two pairs of the joined key ways and the keys) mainly makes contacts. This is so that the roll may be easily broken down by an intensive stress on one pair of the key way and the key.